Spontaneous vortex phases in ferromagnet-superconductor nanocomposites

TL;DR

This paper reports a temperature-induced phase transition between different spontaneous vortex phases in ferromagnet-superconductor nanocomposites, where magnetic nanoparticles generate vortices without external magnetic fields, revealing new vortex dynamics.

Contribution

It introduces the discovery of spontaneous vortex phases caused by magnetic nanoparticles in superconductors and characterizes their phase transitions and dynamics.

Findings

Identification of a temperature-induced phase transition between vortex phases.

Observation of a vortex liquid to vortex glass transition.

Transport measurements support the (H-T) phase diagram.

Abstract

The interplay between superconductivity and magnetism gives rise to many intriguing and exciting phenomena. In this Letter we report about a novel manifestation of this interplay: a temperature induced phase transition between different spontaneous vortex phases in lead superconducting films with embedded magnetic nanoparticles. Unlike common vortices in superconductors the vortex phase appears without any applied magnetic field. The vortices nucleate exclusively due to the stray field of the magnetic nanoparticles, which serve the dual role of providing the internal field and simultaneously acting as pinning centers. As in usual superconductors, one can move the spontaneous vortices with an applied electric current. Transport measurements reveal dynamical phase transitions that depend on temperature (T) and applied field (H) and support the obtained (H-T) phase diagram. In particular, we used a scaling analysis to characterize a transition from a liquid to a novel disordered solid resembling a vortex glass.